You need to have all the cores running at highest clock to get better numbers.
May be you have power governor not set to optimal performance on these machines.
Google for "Linux CPU scaling governor" to get more info on this subject, or
contact your system admin and ask him to take care of the CPU frequencies.

Once this is done, check all the pairs of your machines - ensure that you get
a good number with ib_write_br.
Note that if you have a slower machine in the cluster, general application
performance will suffer from this.